Royal Icing Without Powdered Sugar?

I want to make some sugar cookies for Christmas this year but my Mom has developed some food allergies. I have a recipe for the cookies, but I just realized that there is corn starch in powdered sugar, and corn is a no-no. Is there any way to make a royal-like icing without powdered sugar? Are there any other icings that I might be able to make that avoid cornstarch (and dairy and gluten! but eggs are okay).

I haven't tried it, but i have read that you can make your own powder sugar by running regular granulated sugar through a food processor until it turns to powder. The cornstarch is mostly added to keep the commercial sugar from caking (i believe), so if you use it fresh, it just might work for royal. Worth a google on it at least.